Strategy to scale up microwave synthesis with insight into the thermal and non-thermal effects from energy-based perspective

Highlights

• Strategy to optimize and scale up microwave synthesis via APD/AED parameters.

• Mapping of microwave system heating characteristics by AED.

• AED characterizes reaction completion and temperature.

• APD accentuates microwave non-thermal effect to enhance reaction kinetics.

• Microwave reactor design procedure based on APD and AED.

Abstract

A strategy to industrialize microwave synthesis from the perspectives of process optimization, scale up and reactor design was devised for the synthesis of glycerol carbonate (GC). The strategy is based on the application of absorbed energy density (AED) and absorbed power density (APD) parameters. Here, the microwave synthesis was conducted using a constant-power heating mode. The mapping of microwave heating characteristics based on heating power, reaction temperature and AED was first performed for the microwave system. Then, the optimization of APD and AED was conducted and the intensive optimum conditions at 175 J/g AED and 1.9 W/g APD was used to scale up the microwave synthesis from 0.3 mol (27 g) to 3 mol (267 g). The resulting syntheses were highly reproducible and consistent. Based on APD and AED, the thermal and non-thermal microwave effects can be characterized and they are useful in the design of continuous-flow microwave reactor.

Introduction

Reactive synthesis via microwave system covers a broad spectrum of applications due to its thermal and non-thermal effects to accelerate the heating and the process kinetics [[1], [2], [3]]. However, the scale up of microwave processes, even at the lab scale, remains the primary challenge to viable commercialization. This is in part due to the fact that the strategy to scale up microwave synthesis has yet to be developed and standardized to date. In fact, only a few scientific studies have attempted to scale up microwave synthesis [[4], [5], [6], [7], [8], [9]], by correlating operating parameters such as microwave power, heating time and temperature across process scales. The general strategy often requires ad-hoc re-optimization, adjustment and tuning of the operating parameters when carried out on different scales or at different microwave systems. The haphazard state of microwave scale up strategy calls for a more generalized scale up procedure that includes factors such as heat loss, absorption capability of the reaction system, microwave heat distribution, microwave penetration depth, and the non-thermal effect on the process to achieve a predictable result [10,11].

There are various strategies to scale up microwave synthesis, depending on the modes of heating and operation. For batch operation of microwave systems with constant-power heating, it is intuitive to investigate the applied energy efficiency, i.e. applied power output (or density) and heating time of a microwave system, in order to achieve a certain process performance on various scales [4,7,9]. Upon obtaining the applied energy efficiency data, appropriate operating conditions on various scales can be estimated by interpolation or extrapolation. The applied energy efficiency depends on the geometric characteristics of the resonant cavity and the reactor, the design and effectiveness of the impedance matching circuit to minimize the reflected power, and the dielectric properties of the bulk [9]. The operating condition estimated based on the applied energy efficiency data tends to result in high microwave power output, with a significant reduction in the heating time [9]. Therefore, this scale up strategy is not suitable for handling heat-sensitive components. Alternatively, a better strategy incorporating the effect of microwave absorption capability of reaction system can be considered. In this regard, electromagnetic simulation such as COMSOL is useful to simulate the actual microwave power received by the reaction system, and then to correlate this with the operational setting of the microwave system on various scales [8]. The non-linear behavior of the resulting correlation gives valuable insights on the scale up of microwave reactor. Apart from using sophisticated microwave models and simulations, there is a simplistic energy-based approach to investigate the absorbed power and energy in a microwave system. The energy-based approach involves the calibration of absorbed power density (APD) and absorbed energy density (AED) parameters to characterize the kinetics and the performance of the microwave system on various scales [12,13]. This approach has been successfully developed and implemented in a non-reactive system, i.e. extraction, in our previous studies [12,14,15]. Nevertheless, it requires further modifications and careful adjustment to be adapted into a reactive system. Therein lies the motivation of this study to explore and develop the scale up strategy for microwave synthesis based on APD and AED parameters.

Different from the constant-power heating mode described above, the scale up strategy for temperature-controlled microwave synthesis emphasizes on maintaining the reaction temperature and the heating duration, through the regulation of microwave heating power [5]. The required microwave power to maintain the reaction temperature after ramping is usually low and varies according to the control algorithm of the heating mode [11]. As a result, this heating mode often produce weak microwave specific rate enhancement effect on the synthesis, as compared to that of constant and intermittent power heating modes [11]. Beside, temperature-controlled microwave synthesis can be difficult to reproduce due to the unpredictable process kinetics driven by the inconsistent heating power profile [16].

There are thermal and non-thermal effects that facilitate specific reaction rate enhancement in microwave synthesis [17]. The thermal effect produces volumetric heating, whereas the non-thermal effect involves a direct coupling of microwave energy to the internal energy modes of the reactant molecules [18]. Despite the concept of non-thermal effect in microwave heating being deemed to be controversial in previous days [17], several studies have proven its existence from molecular simulations and analysis [[19], [20], [21], [22], [23], [24], [25]]. The non-thermal effect is able to change the spatial collision and energy distribution of molecules, and to strengthen the interaction between chemical bonds, for better activity and reactivity. For the purpose of process optimization and scale up, many attempted to correlate the microwave non-thermal effect with the reaction kinetics using the Arrhenius equation [[26], [27], [28], [29]]. However, because the enhancement effect could be due to the changes in either the pre-exponential factor, or the apparent activation energy or both, such correlation is not deterministic and difficult to apply [28]. Moreover, the correlation requires data from isothermal reaction, which is not preferable in microwave synthesis as far as reproducibility and kinetic enhancement are concerned, as mentioned previously. Therefore, we propose to characterize the non-thermal effect of microwave synthesis based on the absorbed microwave power and energy and investigate its potential for improving better scale-up and optimization performance.

In this work, we attempt to investigate the non-thermal effect of microwave synthesis of glycerol carbonate (GC) using APD and AED parameters, followed by the development of an optimization and scale up strategy for the process. The synthesis involves transesterification of glycerol with ethylene glycol using heterogeneous catalyst under constant-power heating mode, which is distinct from those reported in isothermal microwave synthesis [[30], [31], [32], [33]]. The outline of this work focuses on the characterization of the energy-based parameters (i.e. APD and AED) during microwave synthesis, the development of the intensive optimum condition for scale up, and the mechanism of the thermal and non-thermal effect from energy-based perspectives.